Words with the suffix -tio we naturally think of as verbals, or nomina actionis, but in the following examples the context makes it fairly certain that they mean the same as their corresponding -mentum nouns.
Formamenta is found only twice: omnia principiorum formamenta queunt in quovis esse nitore, Lucr. 2, 819; si vos fateremini id quod vestra suspicio credidisset formamentis divinis attribuisse, minus erat iniuriae praesumpta in opinatione peccasse, Arn. 3, 16. In the first example, formamenta is used closely following formae and must mean the same thing, the “shapes” of the atoms; in the second example the adjective “divinis” indicates a similar meaning for formamentum; in the following example Vitruvius is giving directions concerning the building of a forum: ita enim erit oblonga eius [forum] formatio et ad spectaculorum rationem utilis dispositio, Vitr. 5, 1. While the directions for the future building might lead us to believe that the word has a predominant verbal force, yet it is just as possible to conceive of it as expressing the result of the process; and this interpretation is even more probable, as the adjective oblonga would properly not be applied to a purely verbal noun.
The verb fodior shows the identity in meaning between fundatio and fundamenta in the following instances: cum fodientes delubro fundamenta caput humanum invenissent, Plin. 28, 2, 4; fundationes eorum operum fodiantur, Vitr. 3, 3. Res Romana and libertas are near enough alike to show that fundamen and fundamentum have the same general meaning in these instances: fundamine magno res Romana valet, Ov. M. 4, 808; haec sunt fundamenta firmissima nostrae libertatis, Balb. 13.
The contexts of hortamen and hortamentum in the two following examples are near enough alike to warrant our saying that the nouns might be interchanged: Decii eventus, ingens hortamen ad omnia pro re publicia audenda, Liv. 10, 29, 5; in conspectu parentum coniugumque ac liberorum quae magna etiam absentibus hortamenta animi sunt, Liv. 7, 11, 6.
There is undoubtedly no more verbal force in the following example of allevatio than in the example of allevamentum, (which is the only one extant): tantis rebus urgemur, nullam ut allevationem quisquam non stultissimus sperare debeat, Fam. IX, 1; Sulla coactus est in adversis fortunis sine ullo remedio atque allevamento permanere, Sulla, 66.
Besides alimentum there are two other nouns, formed on the verb alo, alimonium and alimonia, which also mean support or nourishment, as seen from these parallel examples: plus alimenti in pane quam in ullo alio est, Cels. 2, 18; quid temperatus ab alimonio panis, cui rei dedistis nomen castus?, Arn. 5, 16; amisso omni naturalis alimoniae fundamento, homo exhaustus intereat, Gell. 17, 15, 5.
Although -tus is also usually considered as forming nomina actionis, the example of cruciatus clearly is parallel with that of cruciamentum: confectus iam cruciatu maximorum dolorum, ne id quidem scribere possim, quod...., Att. XI. 11, 1; nec graviora sunt tormenta carnificum, quam interdum cruciamenta morborum, Phil. 11, 4.
Calceamentum, “shoe” or covering for the feet, has two accessory forms, calceamen and calceatus, which are synonymous with it (the former being found only in Pliny): mihi est calciamentum solorum callum, amictui Scythicum tegimen, T. 5, 90; vestitu calceatuque et cetero habitu neque patrio neque civili usus est, Suet. Calig. 52; hinc [sparto] strata rusticis eorum, hinc ignes facesque, hinc calceamina et pastorum vestis, Plin. 19, 2, 7.
The use of ad and a gerund after both invitatio and invitamenta indicate their lack of difference in meaning in these two instances: ad eundem fontem revertendum est, aegritudinem omnem abesse a sapiente, quod inanis sit, quod frustra suscipiatur, quod non natura exoriatur, sed iudicio, sed opinione sed quadam invitatione ad dolendum, cum id decreverimus ita fieri oportere, T. 3, 82; quocirca intellegi necesse est in ipsis rebus, quae discuntur et cognoscuntur, invitamenta inesse, quibus ad discendum cognoscendumque moveamur, F. 5, 52.
Munitio is another -tio noun that ordinarily has verbal force, but not at all infrequently it coincides in meaning with both munimen and munimentum: cum urbem operibus munitionibusque saepsisset, Phil. 13, 9, 20; castella et munitiones idoneis locis imponens, Tac. A. 3, 74. The genitives following munimen and munitio are alike in meaning and function, both being appositional: confisus munitione fossae, B. C. 1, 42, 3; narrat esse locum solidae tectum munimine molis, Ov. M. 4, 771. Munimentum is used of the same kind of “fortification”: fossa, haud parvum munimentum, Liv. 1, 33, 7.